People walked in solemn processions to prayer services marking the anniversary of the worst natural disaster in the nation's history. Many people wore white, a colour associated with mourning in Haiti, and sang hymns as they made their way to the services.

International response

A number of foreign dignitaries are in Haiti for the ceremonies.

Outgoing President Rene Preval and former US President Bill Clinton, who is the UN special envoy to Haiti, attended a ceremony to lay the cornerstone for a new National Tax Office, where many workers were killed last year.

As well as facing the huge task of rebuilding, which has barely begun, Haiti has had to cope with an ongoing cholera outbreak that has so far killed more than 3,500 people, according to government figures.

Political instability has also increased, following November's disputed presidential election.

Some 60 people held a demonstration in central Port-au-Prince, criticizing UN peacekeepers and international aid organisations for their response to the quake, Reuters news agency reported.

Mr Clinton on Tuesday told the BBC of his frustration at the slow pace of reconstruction.

"No-one is more frustrated than I am that we haven't done more," Mr Clinton said during his visit to Port-au-Prince.

But he said he was confident that the speed of reconstruction would pick up.

Several major aid agencies have questioned the effectiveness of the overall response to the earthquake. Medical charity MSF has pointed to a lack of co-ordination, Oxfam said donor countries had too often pursued their own aid priorities, while Merlin said the large number of NGOs has undermined Haiti's own health service.

International donors last March pledged $2.01bn (£1.28bn) for the country's long-term recovery but by the end of December, the amount disbursed totalled $1.28bn - or 63.6%.

Political turmoil

One of the key problems is the continued political uncertainty. November's election was widely denounced as flawed, with reports of fraud and intimidation at polling stations.

Violent protests broke out when the provisional results were announced in December.

The second round was due to take place on 16 January, but has been postponed until next month as there is still no agreement on which candidates will be taking part.

Provisional results put the former First Lady Mirlande Manigat in first place and the government party candidate Jude Celestin in second, just ahead of the pop star, Michel Martelly.

But Mr Martelly insists he won more votes than Mr Celestin and should be in the run-off.

A mission from the Organisation of American States, which was brought in to evaluate the result, is reported to have found in Mr Martelly's favour, but this has not been confirmed.

Mr Preval said on Monday that he had not yet seen the report and has indicated he will only discuss it once the commemorations are concluded.